Interestingly, the streamwise alignment of hairpin vortices near the surface (see figure REF e,f) has been reported
previously by [1]} and [2]} for coaxial round and planar jets, respectively. In spite of the absence of a coaxial phase in our case, we observe the same phenomenon. Additionally, we observe the successive alignment of opposite-signed vortices, where the angle between each vortex pair is determined by the induction effect of the opposite-signed `neighbour' vortex. Similar arrangement of vortices has been reported previously by [3]} for boundary layer turbulence, and [4]} for compressible homogeneous jets. Further details about the specific alignment of the vortical structure is provided below.
